Problem 495. Formatting currency numbers

Given a number, format it properly for textual display using the notation $xxx,xxx,xxx.xx. Assume that no more than two digits will follow the decimal point (i.e. you don't have to worry about rounding). Indicate negative numbers with parentheses.

Examples:

 Input  amt = 34.2
 Output str is '$34.20'

 Input  amt = 91337585
 Output str is '$91,337,585.00'

 Input  amt = -63234922.46
 Output str is '($63,234,922.46)'

 Input  amt = 2784.9
 Output str is '$2,784.90'

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